Debugging During Testing

When writing tests, use the PyCharm "visual debugger" to interactively poke around in the context of a problem.

Primary guardian, great! But what happens if there are no guardians yet? In this step we tackle that problem by combining two great features: visual testing with visual debugging.

Note: In the next tutorial step we do some coding for this situation.


Let's imagine we were working on our test_primary_guardian test and did something like this:

def test_primary_guardian(player_one):
    assert player_one.primary_guardian

It raises an error:

Assertion Error

This error message is quite helpful, but let's imagine you're confused about it. "What do you mean, there's no primary guardian?" You'd like to explore a little to see what's going wrong.

Your print() Will Not Help You Here

What's the number one debugger in Python? Alas, the humble print statement. You thus reach for the "easiest" (wrong!) tool:

def test_primary_guardian(player_one):
    assert player_one.primary_guardian

Your tests run, but...nada. pytest is capturing output. Besides, you've now changed your code as part of investigating a problem. Not smart.

You could learn how to generate console output in pytest, but that would be the same issue -- writing debugging statements into your code which you might accidentally check in.

The debugger was meant for this. Fortunately PyCharm has a great "visual" debugger, tightly integrated with "visual testing." Let's use that.

Breakpoint, Step...Cha-Cha-Cha

Remove the print statement and instead, click in the gutter beside the first line in that test function. This adds a red circle to indicate a breakpoint at assert player_one.primary_guardian:

Set Breakpoint

Let's now run that one test, but under the debugger, which itself will then run pytest. Click the green triangle in the gutter to the left of test_primary_guardian and choose `Debug 'pytest for test_play...'. This brings up a new tool window in the bottom, running the debugger:

Run Under Debugger

Our test execution has stopped on the line with the assertion. Our Variables pane shows that player_one exists in the scope and we can take a look at it:

Expanded Variable in Debugger

Aha, that's the problem! But let's say we still couldn't spot it. We want to poke around interactively.

Highlight player_one.primary_guardian in that line of code -- the one triggering the error -- then right click and choose Evaluate Expression. When you click Evaluate button, you see the output:

Evaluate Expression

You can now poke around interactively in the state at that point by typing in the Expression: field. You can even overwrite the value of a variable in the scope or create new variables.

You're currently poking around in the code for the test, but perhaps you want to poke around in the implementation. Click Step Into and the debugger will put you on the return self.guardians[0] line in the primary_guardian property/method.

With this we quickly see what is the problem: there is no item zero in self.guardians. Finish up by removing the assert player_one.primary_guardian line and clearing the breakpoint by clicking on the red circle. Close the debugger tool window by clicking on its (x).

The debugger is a fantastic tool when doing TDD -- not just when you have a problem, but whenever you want to explore.